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Pressure seems high?

AlwaysLearning2024
AlwaysLearning2024 Member Posts: 35
in THE MAIN WALL

Hi everyone,

1990’s W-M hydronic boiler. Fired up for the cold season and have noticed that the pressure seems high. There is a tick mark on the gauge for roughly 12psi, yet once the water is hot, the pressure rises to about 25psi. I understand the pressure would increase with water temperature yet is the 12PSI the system pressure before heating? Should I relief some of the presssure?


Thanks for any help!

Comments

  • 109A_5
    109A_5 Member Posts: 1,842

    Hello AlwaysLearning2024,

    I would inspect the expansion tank (tap test), the air side of the diaphragm may be flooded due to a leaky

    diaphragm.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
    Mad Dog_2AlwaysLearning2024
  • AlwaysLearning2024
    AlwaysLearning2024 Member Posts: 35
    https://forum.heatinghelp.com/discussion/comment/1821410#Comment_1821410

    Thank you, 109A-5!

    I "tapped" on the expansion tank and it seems to be OK. Should I check the PSI of the expansion tank?

  • pecmsg
    pecmsg Member Posts: 5,452

    Thats the next thing.

  • GGross
    GGross Member Posts: 1,340

    Yes they lose some pressure over time, you want the exp tank pressure close to the cold fill pressure of the system. You have to take the tank off of the system to do this, you cant check the pressure with it connected.

  • HydroAirJoe_2
    HydroAirJoe_2 Member Posts: 6
    edited November 2024
    https://forum.heatinghelp.com/discussion/comment/1821457#Comment_1821457

    Unless the expansion tank has an isolation valve, with a drain, you are not going to get the cold static pressure of the tank. IMHO, every expansion tank should be plumbed with something like this …https://www.nibco.com/nibco-products/valves/ball-valves-combination/h-x167/

    AlwaysLearning2024
  • AlwaysLearning2024
    AlwaysLearning2024 Member Posts: 35

    Thank you! I do have an isolation valve on the expansion tank (simple ball valve), yet not like the one your recommended. Would I still need to let everything cool before closing the valve and testing pressure?

  • Jamie Hall
    Jamie Hall Member Posts: 25,154

    You don't really need to have everything cool before closing the isolation valve, but it's a good idea to do so.

    However just closing the isolation valve is not all that is needed; the tank must not have any water in it when the pressure is set, so you will also need to drain it completely. Do you have a drain valve for the tank? Between it and the isolation valve? If not, you will need to actually physically detach it from the piping…

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • HydroAirJoe_2
    HydroAirJoe_2 Member Posts: 6

    Since it already has an isolation valve, just detach it from the system (careful it will be heavy because there seems to be a lot of water in it). Another approach is to bleed down the system to something like 12lbs, attach a air pump to the tank and pump it up to something like 20 lbs to make sure there is no water in it. Isolate the tank (close the valve), then set the static pressure you want, open the valve

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